The present invention relates to the sterilization and disinfection arts. It finds particular application in conjunction with the detection of peracetic acid concentrations in decontamination systems used for the sterilization or disinfection of medical and pharmaceutical equipment, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to the detection of other oxidizable and reducible species in fluid flow systems.
Endoscopes and similar medical devices having tubes or lumens formed therethrough are being used on an ever increasing basis in the performance of medical procedures. The popularity of these devices has lead to the development of improved decontamination systems, both in terms of the speed of the decontamination process and in the effectiveness of the decontamination. High temperature steam sterilization tends to be destructive towards some of the components of the endoscopes. As a result, liquid sterilization processes have been investigated. Glutaraldehyde, a common liquid disinfectant, is generally effective for disinfection of medical instruments. However, the sterilization process generally takes from 10 to 12 hours, which is frequently too long in today's health care facilities. Another drawback with glutaraldehyde systems is that they sterilize without cleaning. That is, they leave non-living biological contaminants on the medical instruments. The contaminants, although sterile, can break down and liberate harmful toxins when the instruments are subsequently reused.
Recently, peracetic acid sterilization systems have proven effective for the sterilization of medical instruments. Due to its limited shelf life and shipping restrictions, the peracetic acid is often prepared as needed from a mixture of precursors. The peracetic acid precursors are typically mixed with water and other chemicals in the bath. U.S. Pat. No. 5,116,575 to Badertscher, et al. discloses a powdered antimicrobial composition comprising acetylsalicylic acid and sodium perborate. Inhibitors and surfactants are also included in the composition, to aid in cleaning and preventing corrosion of the metal parts of the instruments. The composition is mixed with water in a bath. Items to be sterilized or disinfected are immersed in the bath for a period sufficient to effect sterilization or disinfection. Decontaminated items are rinsed before use to remove traces of the acid and other cleaning chemicals.
To insure effective sterilization or disinfection within a preselected period of time, the concentration of peracetic acid is maintained above a selective minimum effective level, typically around 2300-2500 ppm for sterilization of medical instruments.
The bath sterilization procedures does however have disadvantages. Operator errors can contribute to unsatisfactory sterilization. Specifically, inaccuracy in mixing the components may result in levels of peracetic acid below the minimum level required for sterilization. Thorough cleaning of the instruments is not insured if the instruments are removed from the bath before a minimum exposure period is completed. Handling of the instruments between sterilization and rising stages may lead to recontamination of the instruments.
Recently, dedicated decontamination systems have been developed which sterilize and rinse the medical instruments in an enclosed, automated system. Peracetic acid, either generated in situ or diluted from a concentrate, is delivered to a sterilization vessel and circulated over the instruments to be sterilized. U.S. Pat. No. 5,217,698 to Siegel, et al. discloses an office size instrument sterilization system of this type. Instruments to be sterilized are inserted in a cassette and the sterilant fluid circulated through the cassette. After sterilization, the instruments remain in the cassette in a sterile condition until needed in the hospital. For decontamination of larger instruments, U.S. Pat. No. 5,225,160 to Sanford, et al. discloses a wheeled decontamination apparatus.
To insure that an accurate dose of the powdered sterilants is provided, it is preferable to contain the components in a cup which is opened within the sterilization system when needed. U.S. Pat. No. 5,439,654 discloses a cutter for opening a sterilant-containing cup.
The use of a measured dose of sterilants or precursors does not always guarantee adequate levels of peracetic acid. Peracetic acid precursors and concentrates can decompose over time. Thus, even when the dosage is accurately measured, the concentration for peracetic acid in the solution is not always assured. Further, the peracetic acid concentration may be reduced when the medical instruments are heavily contaminated with biological materials.
Methods have been developed for detection of peracetic acid, and other oxidizable and reducible species, in solution. Dippable papers, for example, are easy to use, but lack accuracy, particularly at concentrations suitable for sterilization or disinfection. Chemical titration methods provide a more accurate measure of peracetic acid in solution, but are time consuming to perform and are prone to operator errors. They do not provide the rapid detection needed for automated sterilization and disinfection systems.
Recently, a number of electrochemical techniques have been developed for detection of oxidizable or reducible chemical species, such as mixtures of peracetic acid and hydrogen peroxide. U.S. Pat. No. 5,400,818 to Consentino, et al. discloses such a sensor. The sensor measures the resistivity of the solution, which is dependent on both the peracetic acid and the hydrogen peroxide concentrations. European patent application EP 0333246 A to Unilever PLC, discloses an electrochemical sensor using an amperometric method, in which a fixed potential is maintained between the reference and the working electrode. The current at the working electrode is used to determine the concentration of peracetic acid. Other species present, however, influence the current flowing, and hence the accuracy of the results.
U.S. Pat. No. 5,503,720 to Teske discloses a process for the determination of reducible or oxidizable species, such as peracetic acid, in sewage waste. The process uses potentiostatic amperometry to detect peracetic acid concentrations. The technique, however, depends on the achievement of a steady state, which frequently takes several hours. Such a detection system is unsuited to a fairly short term sterilization process.
The electrode systems used in such detection systems are generally bulky devices which are both costly to manufacture and require careful preparation of the electrode surfaces prior to use. Prolonged exposure to the test solution tends to degrade the electrodes and thus recalibration is generally employed before each use. Inaccurate measurements result when recalibration is not carried out or the electrodes are not thoroughly cleaned.
Recently disposable electrodes have been developed for analytical assays in medical and biochemical samples. The electrodes are laid down as an ink onto a plastic card. Leads connect the electrodes to electrochemical monitoring equipment. Typically, a few drops of the solution placed on the electrode. However, there is generally no method of determining when the electrode has reached the end of its useful life and for preventing reuse of an electrode intended for a single use.
The present invention provides for a new disposable plastic sensor card for peracetic acid and associated reader and sterilization equipment which overcomes the above-referenced problems and others.